1. Field of the Invention
The present invention relates to a paper roll feed mechanism capable of correcting skew of paper unwound from a paper roll, a paper roll feed cassette using the paper roll feed mechanism, and an image forming apparatus using the paper roll feed mechanism and the paper roll feed cassette.
2. Description of the Related Art
A thermal printer using a thermal head opposed to a platen is known as one of image forming apparatuses. As digital cameras become increasingly popular, rolls of paper are widely used as recording paper serving as printing media for thermal printers. A roll of paper (i.e., a paper roll) is cost-effective in high-quality photographic printing.
A paper roll includes a long sheet of recording paper wound in a cylindrical shape. Paper rolls are classified into two types: The first type is a paper roll including paper wound on a paper tube, serving as a winding core. The second type is a paper roll including paper wound with no winding core (the second type paper roll will be called a core-free paper roll hereinafter). Both the types are used as follows: A necessary amount of paper is unwounded from a paper roll and is moved to a thermal head every printing. After printing, the printed paper is cut into a sheet having a predetermined size.
In a thermal recording technique of directly printing an image onto thermosensitive paper of a paper roll, respective colors of thermosensitive color-developing layers formed in the paper are sequentially developed, thus forming a full-color image.
In a thermal transfer recording technique of printing an image on a paper roll using ink ribbons of three colors, i.e., yellow, magenta, and cyan, solid inks on the respective ink ribbons are thermally transferred to paper, thus forming a full-color image.
Such color-capable thermal printers include a three-pass one-head type and a one-pass three-head type. In the three-pass one-head type thermal printer, paper feed and return is repeated three times in such a manner that paper unwound from a paper roll is moved forward and backward so as to pass through nip between one thermal head and a platen, thus printing a color image on the paper. In the one-pass three-head type thermal printer, paper unwound from a paper roll is moved once so as to pass through nip between each thermal head and a platen, thus printing a color image. As for monochrome printing, a one-pass one-head type thermal printer may be used.
In printing using a thermal printer, therefore, paper unwounded from a paper roll by a paper feed roller is allowed to pass through nip between a thermal head and a platen three times or once. To allow the paper to pass therethrough, it is necessary to move the thermal head away from the platen, i.e., retract the thermal head. In other words, in the thermal printer, the thermal head is temporarily retracted and the paper is fed. After the paper reaches under the thermal head, the thermal head is pressed against the paper (or the ink ribbon on the paper) on the platen and the paper (or the ink ribbon on the paper) is conveyed while heating elements of the thermal head are generating heat, thus printing an image on the paper.
After printing, the thermal head is again retracted, the printed paper is fed and cut into a sheet with a predetermined size, and the sheet is ejected. In the three-pass one-head type, after printing, the thermal head is retracted, the paper is returned, and printing is again performed. This operation is repeated.
As described above, paper unwound from the paper roll is fed to the thermal head every printing, and after printing, the printed paper is cut into a sheet. Disadvantageously, if paper is not unwounded straight from a paper roll, i.e., the paper is skewed, the skew affects subsequent operations, causing paper jamming or poor printing.
To overcome the disadvantages, various techniques of preventing skew are proposed. For example, Japanese Unexamined Patent Application Publication No. 2005-262585 (Patent Document 1) discloses a printer using one of the various techniques. In this printer, a spring-urged pressure aligning plate is brought into contact with one side edge of paper unwounded from a paper roll to press the paper against an opposed fixed aligning plate, thus correcting skew.
Japanese Unexamined Patent Application Publication No. 9-255196 (Patent Document 2) discloses a deskewing structure using a U-shaped bar. This bar is put on the outer surface of a paper roll such that both sides of the bar control the position of the paper roll in the widthwise direction thereof. In this deskewing structure, if the outer diameter of the paper roll is changed, the bar is swung in response to the change in outer diameter. The position of the paper roll in the widthwise direction, therefore, is continuously controlled, thus preventing skew of paper.
Japanese Unexamined Patent Application Publication No. 2002-53231 (Patent Document 3) discloses a paper feed cassette capable of deskewing sheets. A pressure contact member having an inclined contact surface is spring-urged against side edges of cut sheets stacked in the cassette, thus preventing skew of each sheet. Since the pressure contact member is in contact with the uppermost cut sheet, the sheet is prevented from skewing even when the amount of remaining sheets is small.